1. Field of the Invention
This invention relates to electrodes for electrochemical cells such as electrolytic and galvanic cells.
2. Description of the Prior Art
Fuel cells are devices for directly converting the chemical energy of a fuel into electrical power. Generally, a fuel cell comprises two gas diffusion electrodes (an anode and a cathode) and an electrolyte impregnated matrix between the two electrodes. A catalyst layer is present on the electrolyte facing surface of each electrode. In operation, a typical fuel cell is fed with a hydrogen containing gas at the anode and an oxygen-containing gas is fed to the cathode. The gas is diffused through the electrodes to react at catalyst sites to yield water, heat and electrical energy. On the anode side of the fuel cell, hydrogen is electrochemically oxidized to give electrons. The electrical current so generated is conducted from the anode through an external circuit to the cathode. On the cathode side of the cell, the electrons are electrochemically combined with oxygen. A flow of ions through the electrolyte completes the circuit.
There is a constant search for ways in which to improve fuel cell performance. Even slight increases in performance can make the difference between a fuel cell which fills specific requirements in comparison with one which does not. In addition, there is a constant search for ways in which the cost of producing the elements of a fuel cell can be reduced. Specifically, the methods of fabricating fuel cell electrodes have involved various procedures which generally are not suitable to automated production.
In U.S. Pat. No. 4,272,353, a method of making electrodes using a solid polymer electrolyte base member and catalyst is disclosed. The catalyst is deposited upon the surface of the solid polymer electrolyte base member, the surface of which has been previously toughened by suitable abrading means. The catalyst is in the form of metal particles which are subsequently fixed upon the toughened surface by the use of pressure, heat, adhesive, binder, solvent, electrostatic means, etc.
In U.S. Pat. No. 4,810,594, fuel cell electrodes are prepared by the deposition of a hydrophobic polymer and electrocatalyst layer on the surface of a porous electrode substrate which is thereafter press-sintered. Carbon fiber paper substrates are preferred in the preparation of the fuel cell electrodes. The catalyst is a catalyzed carbon particle prepared from a noble metal which is dispersed upon graphitized carbon black. The catalyzed carbon particles are blended with a hydrophobic polymer binder in an aqueous mixture. Prior to coating the catalyst/binder mixture onto the carbon fiber paper substrate, the aqueous suspension of catalyzed carbon particles is flocculated, for example, by heating or by the addition of a flocculating agent. Other references showing the preparation of electrodes comprising carbon fiber paper are: U.S. Pat. Nos. 4,349,428; 4,248,682; 4,647,359; and 4,293,396.
In U.S. Pat. Nos. 3,432,355 and 3,297,484, electrodes for fuel cells are disclosed which are prepared by first casting a film from an aqueous emulsion of polytetrafluoroethylene onto a casting surface, sintering the polytetrafluoroethylene and, thereafter, spreading thereon a film of a mixture of catalytic metal particles and polytetrafluoroethylene. This process necessitates a batch system of preparation.
In U.S. Pat. No. 4,166,143, fuel cell electrodes are disclosed as being prepared by the application of a flocculated mixture of a platinum-on-carbon electrocatalyst in admixture with a dispersion of polytetrafluoroethylene.
In U.S. Pat. No. 4,752,370, membrane/electrode assemblies are disclosed in which catalyst particles in admixture with a binder are deposited upon an ion exchange membrane from a slurry of catalytically active particles, optionally containing a binder. The binder can be a fluoropolymer, such as polytetrafluoroethylene and the solution/dispersion can be formed utilizing a solvent for the particularly preferred ion exchange fluoropolymer utilized as a binder. The catalytically active particles are disclosed as being applied to the ion exchange membrane utilizing metering bars, metering knives, or metering rods. Usually, the coatings on the ion exchange membrane of the catalyst are built up to the thickness desired by repetitive coating application.